I finally finished this game (PC version) after struggling with it for the last two weeks. Here are some thoughts.
Gameplay: The stealth action is fun, but heavily dumbed down from the previous SC games. Chaos Theory's elaborate light and sound meters have been replaced by a glowing circle that is either green (invisible) or yellow (visible), and sometimes does not even correlate with the actual light level that is visible. You don't get the night vision goggles in many levels and there is no health meter (your health recharges). The level design is a mixed bag. Some levels are outstanding while others are pretty mediocre. Apart from the JBA levels, it's somewhat linear compared to CT, although not overly so. The much-hyped trust system is interesting but doesn't play any serious part in the game until near the end.
Story: Pretty interesting and certainly above average, but has some plotholes and generally lacks the depth of what the previous games had. There is only one story-related cutscene in the entire game, and a lot of things near the end are not fleshed out well. There are three endings, although I only had the patience to go through one (see below).
Stability: Atrocious. The sheer number of bugs in this game defies belief. Sporadically crashes for no reason throughout the game, and it seems to be impossible to complete it at all without using various workarounds. At one point, the game was crashing every time I ran into a (mandatory) autosave checkpoint, and I only got around it by adding the Unreal engine's ghost command into the game's ini files to fly around it. There was another level where fallen enemies came back to life as soon as you stopped looking at them, and one more where loading a savegame caused a crash, so you had to run through it without saving. A look at the Ubi forums indicates that I got rather lucky and most people have encountered much more trouble. Many can't get the game to run at all. Ubi has reportedly dropped support for this game and there will almost certainly be no more patches.
Graphics: A mixed bag. Excellent character models (Fisher's model is probably the best I've seen in any game) and the superb parallax mapping from CT appears in some areas, but other places look like they're from a five year old game. The HDR and shadows are inferior to those in CT. Requires an SM3 card. Very poorly optimized; it looks a bit worse than CT overall but somehow still manages to run at a third of the framerate. I found it choppy but playable at 1024x768 without AA on an X1900XTX.
Audio: Some positional audio glitches, but very good overall. The dynamic music is nicely implemented. I like how the music sharply increases in volume as you creep behind a guy.
Interface: One of the worst examples of consolization I've seen in a long time. I can think of around 15 different glitches in the menus. The various minigames have all kinds of strange control problems and the mouse cursor randomly vanishes in some menus. You can't name savegames and it puts them in a random order in the menu, so you have to rely on the dates/times to identify them. Many of the key mappings inexplicably can't be changed. Makes you watch an unskippable and irritating 30 second movie every time you go into the main menu, although this can be fixed by replacing the movie file.
Multiplayer: Didn't try this, as it seems hardly anyone has actually gotten it working properly.
Overall: 3/10. It has its cool moments and can be fun, but only if you have a lot of patience. It has way too many bugs to be recommended to anyone. A strictly mediocre game by itself, and utter trash in comparison to the highly polished CT.